Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

3. Acids and Bases

Ranking Acidity

Organic Chemistry

3. Acids and Bases

Ranking Acidity

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Problem 51a

Textbook Question

Identify the most stable conjugate base in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer.
(a)

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Analyze the electronegativity of the atoms bearing the negative charge. Oxygen (O) is more electronegative than sulfur (S), meaning it can better stabilize a negative charge.

Consider the inductive effect of the trifluoromethyl group (CF₃) attached to the oxygen atom. The CF₃ group is highly electronegative and withdraws electron density through the sigma bond, further stabilizing the negative charge on the oxygen.

Compare the size of the atoms. Sulfur is larger than oxygen, which allows the negative charge to be spread over a larger volume, providing some stabilization. However, this effect is less significant compared to the electronegativity and inductive effects.

Evaluate the overall resonance and delocalization possibilities. In this case, neither structure has resonance stabilization, so this factor does not contribute to the stability comparison.

Weigh the combined effects of electronegativity and inductive stabilization (stronger in the CF₃O⁻ structure) against the size-based stabilization of sulfur. The CF₃O⁻ conjugate base is more stable due to the dominance of electronegativity and inductive effects.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Conjugate Bases

A conjugate base is the species that remains after an acid donates a proton (H+). The stability of a conjugate base is crucial in determining the strength of its corresponding acid; stronger acids have more stable conjugate bases. Factors influencing stability include electronegativity, resonance, and the size of the atom bearing the negative charge.

Resonance Stabilization

Resonance stabilization occurs when a molecule can be represented by multiple valid Lewis structures, allowing the negative charge to be delocalized over several atoms. This delocalization reduces electron density on any single atom, enhancing stability. In evaluating conjugate bases, resonance is a key factor, as more resonance structures typically lead to greater stability.

Electronegativity and Charge Distribution

Electronegativity refers to the ability of an atom to attract electrons in a bond. In the context of conjugate bases, a more electronegative atom stabilizing a negative charge will lead to a more stable conjugate base. When analyzing pairs of conjugate bases, the distribution of charge relative to electronegativity helps determine which base is more stable, as negative charges are better accommodated by more electronegative atoms.

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Related Practice

Textbook Question

Identify the stronger base in each pair. Explain your choice. Citing pK_a values is not an acceptable answer.

(d)

Textbook Question

Imidazole is a heteroaromatic base. Which nitrogen, a or b, is most basic?

Textbook Question

Without using pK_a values, pick out the least reactive (most stable) base in each pair. Explain your answer.

(b)

Textbook Question

Using qualitative reasoning for the acid–base reactions shown,

(i) which is stronger, the acid or the conjugate acid?

(ii) Which side of the reaction is favored?

(iii) Would you expect a K_eqgreater than, equal to, or less than 1?

(b)

Textbook Question

Identify the most stable conjugate base in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer.

(f)

Textbook Question

Identify the most acidic proton in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer. [Always start by drawing the conjugate base.]

(b)

Textbook Question

Identify the most acidic proton in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer. [Always start by drawing the conjugate base.]

(d)

Textbook Question

Which acid in each pair would you expect to more readily donate a proton to a basic compound?

(a)

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